for Google+ non-business page:

• If there is a custom URL available for your account, you will see a banner below your name on your Plus profile.
• Click the link in the banner.
• You’ll see the custom URLs you can choose from. You may also need to add a few letters or numbers in the text box to make it unique to you.
• Next to I agree with Terms of Service click the checkbox.
• Click Change URL.
• When you’re ready to add your custom URL to your profile, click Confirm.

Rules for getting a custom URL:

There are a few rules to keep in mind when you’re trying to get a custom URL. If you’re having trouble, make sure:

• You have ten or more followers (people who have added you to their circles)
• Your account is at least 30 days old
• Your profile has a profile photo
• Your account is in good standing
• You do not try to get 4th custom URL in past 2 years with your name

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for Google+ business page:

• Sign in to Google My Business
• Click Business Pages at the top of the page, then click Manage this page for the page you'd like to manage
• Click the three dash menu and choose Google+ page
• Click the About tab, and under Get your custom URL, click Get URL. You won't see this option if your page doesn't meet the eligibility requirements.
• You’ll see the custom URL you’ve been approved for, which you aren’t able to change. You may also need to add a few letters or numbers to make it unique to you.
• Check the box next to I agree to the Terms of Service. Click Change URL in the bottom left corner
• You may be asked to verify your account using your mobile phone number. To do this, you’ll need to:
  • Enter your mobile phone number when prompted, and click Send code in the bottom left corner
  • Check your phone for the code that was sent to you
  • Enter that code in the box on your screen, then click Verify in the bottom left corner
• Click Confirm choice to permanently add your URL to your profile